In a plain old telephone service (“POTS”) system network using tip ring wire pair (“TR”) technology, it is desirable to protect subscriber line interface circuits (“SLIC”), an example of which typically interfaces a plurality of telephone lines with a telephone exchange network, from over voltage and over current conditions. Such over voltage and over current conditions may be the result of lightning surges, network system malfunctions or other occurrences that cause the rapid input of energy onto a TR pair. In addition, it is desirable to prevent SLICs from conducting and/or radiating electromagnetic interference (“EMI”) energy.
Conventional protection device typically use either line feed resistor (“LFR”) networks and/or relay devices to protect against over current and over voltage conditions. These devices may provide adequate protection to the SLIC devices, but the circuit board space they use and the cost they contribute to the overall cost of a SLIC card are not insignificant. In addition, LFR are typically thru-hole devices that present not insignificant manufacturing costs and challenges when a SLIC card is manufactured using surface mount technology (“SMT”) processes.
Some attempted solutions also use switching devices, such as thyristors, for example, to shunt over voltage or over current impulses to ground when a certain predetermined energy threshold is exceeded by the over voltage or over current impulse(s). These solutions are effective, but conventionally use at least two thyristors for each TR pair. Since each thyristor occupies space on a circuit board and requires a certain amount of connection complexity to integrate into a circuit board that serves a plurality of TR pairs, the cost of the multiple thyristors used in such an arrangement is not insignificant.
With respect to EMI filtering, conventional techniques typically use capacitors of selected values to have resonant frequencies in the neighborhood of 100 MHz. While these filters provide adequate filtering of EMI, they also adversely affect voice signals having frequencies in the range of 200-3400 Hz.
Thus, there is a need in the art for protection circuitry that protects a plurality of TR pairs without the need for LFR networks or relays and that uses fewer thyristors than are used in a conventional thyristor based protection circuit. Furthermore, there is a need for a protection circuit that filters EMI without adversely affecting the quality of voice signals.